1. Field of the Invention
The present invention relates to a photothermographic material, and particularly to a photothermographic material suitable for medical imaging, industrial photographic imaging, graphic arts and COM.
2. Description of the Related Art
In recent years, dry photographic development processing has been strongly required in fields of medical imaging, graphic arts, etc. from the viewpoints of environmental preservation and space saving. In that fields, digitalization has been in progress and systems have been expanded their applications rapidly, in which image information is scanned into a computer, saved, modified if necessary, moved to a desired place via data communications and outputted on a photosensitive material by a laser image setter or a laser imager, and then the photosensitive material is developed to provide an image. The photosensitive material is required to record the image information by laser exposure with high intensity and to form a clear black-toned image with high resolution and sharpness. As such digital recording imaging materials, various hard copying systems using a pigment or a dye such as inkjet printers and electrophotographies have been distributed for common image formation. However, the systems are insufficient in image qualities (sharpness, granularity, gradation and color tone) and a recording speed (sensitivity) for the medical imaging, and do not attained a level that they are alternative to conventional, wet-developing silver salt films for medical use.
Photothermographic imaging systems using an organic silver salt are described in U.S. Pat. Nos. 3,152,904 and 3,457,075; and D. H. Klosterboer, Thermally Processed Silver Systems in Imaging Processes and Materials, Neblette 8th Ed., Edited by J. Sturge, V. Walworth and A. Shepp, Chapter 9, Page 279, 1989.
Photothermographic materials generally comprises a photosensitive layer, in which a photosensitive silver halide, a reducing agent, a silver salt such as an organic silver salt that can be reduced, and an optional agent for controlling color tone of developed silver are dispersed in a matrix of a binder. Used as the binder are polymers having a glass transition point lower than the heat-developing temperature. In general, polyvinylbutyral is used as the binder, and an image-forming layer is formed by the steps of: dissolving the binder in an organic solvent such as methyl ethyl ketone (hereinafter referred to as MEK); dispersing or dissolving the photosensitive silver halide, the reducing agent, the organic silver salt, etc. therein; applying the resultant to a support into a film; and drying the film. Also photothermographic materials using polymer latex as the binder have been developed in recent years.
When the photothermographic materials are heated at such a high temperature as 80° C. or more after exposure, a black-toned silver image is formed by a redox reaction between the reducing agent and the silver halide or the silver salt that can be reduced as an oxidizing agent. The redox reaction is accelerated by catalytic activity of a silver halide latent image formed by the exposure, and as a result, the black-toned silver image is formed in the exposed region. The photothermographic materials are disclosed in many references including U.S. Pat. No. 2,910,377 and JP-B No. 43-4924. However, the photosensitive silver halide, the organic silver salt, the reducing agent, etc. are remained in the photothermographic materials even after the thermal developing, whereby the photothermographic materials are seriously disadvantageous in that printout and fogging is increased during storage of the image.
Generally used as the laser are gas lasers (Ar+, He—Ne, He—Cd), YAG lasers, dye lasers, semiconductor lasers, etc. The semiconductor lasers may be used in combination with a second harmonic-generating device. Emission wavelength region of the laser may be in a wide range of blue to infrared region. Among the lasers, infrared semiconductor lasers can stably emit light with reduced costs to be particularly suitable for designing a laser image output system, which is compact and excellent in operationality and which can be used in any installation location with ease. The photothermographic materials are thus required to have infrared photosensitivity, and various efforts have been made to increase the sensitivity to the infrared radiation. However, infrared spectral sensitizing components are generally unstable and decomposed during the storage of the photosensitive material to reduce the sensitivity. Thus, the photosensitive materials with infrared spectral sensitization have been required to be improved in storage stability in addition to the sensitivity.
Organic solution application-type photosensitive materials, particularly such that uses polyvinylbutyral as the binder, result in a remarkable change of the sensitivity during the storage as compared with aqueous solution application-type photosensitive materials using the polymer latex. This instability was considered to be caused by a residual organic solvent from comparing both. Under the circumstances, technologies for increasing the sensitivity with excellent storage stability are required, particularly in the case of using the organic solvent for application.